Tungsten exhibits exceptional resistance to environmental degradation, maintaining its structure even in harsh environments like salt water. The simple answer to whether tungsten rusts is no, because “rust” is chemically specific to iron. As a refractory metal, tungsten possesses inherent properties that prevent the destructive oxidation seen in common metals. This stability makes it a material of choice for demanding applications involving long-term exposure to corrosive elements.
Tungsten and the Chemistry of Corrosion
Tungsten is classified as a refractory metal, characterized by extremely high melting points, high density, and superior resistance to heat and wear. Its chemical properties set it apart from metals like iron, which are prone to rapid degradation. This distinction is important because the terms “rust” and “corrosion” are often mistakenly used interchangeably.
Corrosion is the general term for the deterioration of a material due to chemical or electrochemical reaction with its environment. Rust, however, is a very specific type of corrosion that occurs only when iron or its alloys react with oxygen and moisture. The reddish-brown substance formed is iron oxide, a compound unique to iron-based materials.
Since tungsten is not an iron-based metal, it cannot form iron oxide, making it chemically impossible for pure tungsten to rust. While tungsten can undergo oxidation (reaction with oxygen), the product is chemically distinct from rust. The metal’s behavior in environments containing oxygen and water, such as salt water, leads to a protective outcome.
The Protective Mechanism of Tungsten
Tungsten’s resistance to chemical attack stems from a self-passivation process that occurs upon initial exposure to an oxidizing environment. When the metal surface meets oxygen, it quickly forms a thin layer of tungsten trioxide (\(WO_3\)). This passive layer acts as a highly effective barrier, sealing the underlying metal from further reaction.
This tungsten oxide layer is stable and non-porous, meaning it does not flake off or expose fresh metal beneath, unlike iron rust. The protective film is chemically inert under standard conditions, preventing oxygen and water molecules from reaching the pure tungsten below. The metal’s inherent stability, reinforced by its high melting point of 3,422°C, further contributes to the integrity of this oxide layer.
The formation of this hard, tenacious oxide film is the key to tungsten’s long-term survival in corrosive settings. It effectively halts the oxidation process at the surface, contrasting sharply with the continuous, destructive cycle of iron rusting. This stable \(WO_3\) film provides a lasting defense against the elements.
Tungsten’s Performance in Saltwater
Saltwater presents an aggressive environment for most metals due to dissolved oxygen and a high concentration of chloride ions. While chloride ions penetrate and break down protective oxide layers on many metals, tungsten resists this challenge effectively. The passive tungsten oxide film maintains its integrity even when immersed in seawater.
Tungsten and its alloys are considered inert in neutral or alkaline aqueous solutions, which includes the typical pH range of ocean water. In marine engineering, materials containing tungsten, such as tungsten carbide, are employed for components like propeller shafts and specialized bearings. Tungsten additions are also used to enhance the corrosion resistance of super duplex stainless steels in seawater environments.
Measurable degradation of tungsten typically only occurs under extreme conditions not found in the natural marine environment. The metal maintains its stability unless subjected to very strong acids, highly concentrated oxidizing agents, or temperatures exceeding \(400^\circ C\). For practical applications in the ocean, tungsten offers complete resistance to the corrosive forces of salt water and is immune to pitting corrosion.